Present Stirling engine designs have highly pressurized gas, such as hydrogen, utilized as the working gas. The basic principle of Stirling engine operation is the utilization of a piston disposed between a high pressure and low pressure regions which results in the movement of the piston which provides a work output. In an unlubricated Stirling engine, a non-metallic material is typically used for the piston ring or rings to maintain a seal between the two pressure regions. Since such material normally has a low modulus of elasticity, in order to maintain contact between the ring and the cylinder wall for sealing purposes, a radial force must be applied to the ring. This has been usually achieved by connecting the inner surface of the piston ring with the high pressure gas in the engine. Some designs provide additional loading by various forms of internal expanders. However, with such arrangements, the contact pressure between the ring and the cylinder will vary during each engine cycle and with engine operating conditions. This varying contact pressure leads to inefficient sealing of the piston in the cylinder which is necessary for effective operation.
In addition, while it is important to maintain contact for sealing purposes, it is also important to limit and control the contact pressure to reduce the friction and wear of the rings.